The present invention relates to a vacuum control valve for automatically varying value of negative pressure from a vacuum source such as a vacuum pump. The invention relates in particular to a vacuum control valve used with an exhaust gas recirculation valve (hereinafter referred to as EGR valve) and for controlling the negative pressure of the vacuum applied to the EGR valve.
Recently it has become necessary to purify exhaust gases from Diesel engines and many experiments have been made for the purpose. One of the difficulties for Diesel engines is that a vacuum source, such as an intake negative pressure representing an operational load of an engine, is not present in Diesel engines. Therefore it is necessary for Diesel engines to provide a vacuum pump driven by the engine and a vacuum control valve for modulating the negative pressure from the vacuum pump in order to produce such a negative pressure as representing an operational load of the engine.
Such a vacuum control valve is proposed in a Japanese prior application (Japanese Utility Model Application No. 55-127027, published on Mar. 19, 1982 under Publication No. 57-49548). In the known vacuum control valve, a preset load for a diaphragm is changed by an angular rotation of a cam which is linked with an acceleration pedal or a control rack of a fuel injection pump for a Diesel engine, so that a negative pressure representing an operational load of the engine can be obtained. However, this known vacuum control valve has the following drawbacks: A life of a cam or a cam follower is relatively short, since a diameter of the diaphragm is relatively large and thereby a spring constant of a spring is large and accordingly the cam follower is in contact with the cam by a large contact force. When the diameter of the diaphragm is reduced to decrease the spring constant, and also when the diameter of the cam is reduced correspondingly, a slope of a cam surface becomes larger for obtaining the same amount of an axial movement of the cam follower, and thereby a smooth movement of the cam follower on the cam surface is prevented.